The present invention relates to a system for digitizing the trajectory of the intended target point of a moving beam wherein the beam has a distributed intensity on a surface at which it is directed.
A typical analog oscilloscope produces a waveform representing the time varying behavior of an input signal by utilizing the input signal to control the vertical position of an electron beam as it sweeps horizontally across a phosphorescent screen. After the beam strikes phosphors on the screen the phosphors glow for a time, and if the persistence of the phosphors is long enough in relation to the sweep rate of the beam, the glowing phosphors show the trajectory ("trace") of the beam across the screen, thereby providing a waveform display representing the behavior of the input signal. However, inasmuch as the phosphors in many oscilloscopes glow only for a brief period of time after excitation by the beam, the waveform display must be refreshed by repetitive sweeps of the beam across the screen in order for the waveform to be observed. Such oscilloscopes are typically utilized to monitor periodic input signals which can control the vertical position of the beam in the same way during each sweep so that a similar waveform is displayed in response to each sweep.
Some analog "storage" oscilloscopes utilize phosphors which are highly persistent so that a waveform produced in response to a single sweep of the beam is displayed for relatively long time. Storage oscilloscopes are often used to capture and display no-repetitive signal bursts, but the display produced by such an oscilloscope eventually fades. Storage oscilloscopes have been fitted with photographic cameras which permanently record the waveform display; however, photographic film is expensive and may require an inconvenient amount of time to develop and print.
It is often desirable to digitize a waveform so that its characteristics can be analyzed by a digital computer and so that information about the waveform can be compactly and inexpensively stored in digital data storage media. The Tektronix model DCS01 Digitizing Camera System employs a video camera which may be aimed at an oscilloscope screen, the camera system utilizing a charge-coupled device (CCD) to produce an array of digital data representing the light intensity distribution over the surface of the oscilloscope screen. The CCD comprises a closely spaced array of MOS capacitors. Each capacitor, under controlled bias conditions, acquires charge in proportion to the amount of light striking the capacitor during a sampling period, and the voltage developed by any capacitor of the array is thus proportional to the time-averaged light intensity of a corresponding portion of the oscilloscope screen during the sampling period. The charges built up in the capacitors during the sampling period are subsequently shifted from capacitor-to-capacitor along rows of the capacitor array to provide voltage signals at output terminals of the device. These voltage signals are multiplexed to produce an RS170 standard video signal. The video signal is then digitized by an eight-bit analog-to-digital converter into a 490.times.480 element digital data array representing the light intensity distribution of the waveform display.
At any instant during its sweep across the oscilloscope screen, the beam is nominally directed at a target point on the screen at horizontal and vertical positions on the screen representing the current magnitudes of the sweep and input signals, respectively. However, the electron beam has a non-uniform, two-dimensional intensity distribution in the plane defined by the surface of the screen, and typically an area of the screen surrounding the target point is illuminated, not just the target point itself. The intensity distribution of the beam, which may or may not be substantially Gaussian, nonetheless typically has a relatively more intense central portion and a less intense peripheral portion, and as the beam sweeps across the screen it produces a "feathered" trace in which phosphors near the center of the trace glow more brightly than phosphors near the edge of the trace. When the trace is subsequently "captured" in the form of a charge distribution within the CCD of the camera system, and then converted into the 490.times.480 element data array, the resulting data array represents the two-dimensional light intensity distribution on the screen.
The 490.times.480 element intensity data array could be stored and utilized directly to control a 490.times.480 pixel computer-generated display wherein the value of each data word determines the intensity of a corresponding pixel. In such case, a computer-generated waveform display based on this intensity data would have an appearance substantially identical to the original waveform displayed on the oscilloscope screen and would mimic the feathered intensity distribution of the trace forming the original waveform display on the oscilloscope.
However, it is inconvenient and expensive to provide enough memory to store 490.times.480 eight-bit data words for each waveform to be stored. Moreover, it is not particularly desirable to display a waveform having a feathered trace inasmuch as an operator typically wishes to view a waveform as a fine, distinct line representing the trajectory of the intended target point of the beam as the beam sweeps across the screen. In addition, when the data is to be analyzed by a digital computer to determine characteristics of the waveform such as peak value, period, rise time and the like, it is difficult for the computer to ascertain such characteristics from waveform data presented in the form of an intensity distribution data array.
The 490.times.480 element intensity data array may be thought of as 490 sequences of 480 words, each sequence representing the intensity distribution along a separate vertical axis of the oscilloscope screen. To present waveform data in a more compact and useful form, each one of the data sequences indicating intensity distribution along a corresponding vertical axis on the oscilloscope screen may be converted by a digital computer to a single data element indicating a single vertical position on that axis. Ideally, that vertical position should represent the intended target point of the beam along that axis when the beam moved across the axis, the "intended target point" being the point on the oscilloscope screen that represents the actual magnitude of the input signal at the time (or sweep signal magnitude) represented by the vertical axis. Since there are 490 intensity data sequences, a single waveform magnitude data sequence of 490 data elements would be produced. The computer may then produce a waveform display by illuminating a line of 490 pixels across a CRT screen, the vertical position on the screen of each successive pixel of the line being determined by the magnitude of a successive one of the elements of the waveform magnitude data sequence, the intensity of the illuminated pixels being uniform. Thus the line ("trace") forming the computer generated waveform is not several pixels wide with each pixel having a different intensity (as would be the case when the intensity data array is utilized to directly control the waveform display), but rather is only a single pixel wide and pixels in the trace are uniformly illuminated.
FIG. 1 shows an idealized representation of light intensity distribution of a typical trace as might be produced on an analog oscilloscope screen in response to a square wave input signal, the trace being magnified to more clearly show the light intensity distribution. The darkest portions of the trace represent areas of highest light intensity and the lightest portions of the trace represent areas of lowest light intensity. Since the beam has a non-uniform intensity distribution and moves at non-uniform rates across the screen, the trace has non-uniform intensity. At waveform peaks 10, where the beam traverses the screen horizontally at a relatively slow rate, the light intensity of the trace is highest particularly in interior portions 12 of the trace produced by high intensity interior portions of the beam. The trace intensity is lower near its periphery 14 produced in response to low intensity peripheral portions of the beam. On the leading and trailing edges 16 of the waveform, the light intensity of the trace is very low because in these areas the beam moves rapidly and does not supply enough energy to substantially excite phosphors on the screen.
FIG. 2 is a diagram of light intensity distribution along vertical axis 2--2 of the display of FIG. 1. The vertical dotted lines of FIG. 2 represent positions along axis 2--2 corresponding to the centers of screen areas sensed by separate capacitors of the CCD along a capacitor array axis corresponding to axis 2--2. Each CCD capacitor measures a light intensity substantially equal to the intensity shown on the curve of FIG. 2 where a corresponding dotted line crosses the intensity curve. In the example of FIG. 2, the intensity distribution of the trace along axis 2--2 is somewhat bell-shaped although not Gaussian since in this example the intensity distribution of the beam does not happen to be Gaussian. The CCD device represents this distribution as a set of voltage levels which are converted into a sequence of 490 digital data values. For the intensity distribution along axis 2--2 of FIG. 2, only 11 of the 490 digital data values will be non-zero.
As previously discussed, it is desirable to convert this sequence into a single data value indicating the intended target point of the beam as it bisected axis 2--2. However, to determine an intended target point of the trace along the axis based on the intensity distribution requires an assumption to be made as to how the intended target point is related to the intensity distribution. Under the assumption that at any given time the most intense portion of the beam is directed to the target point, one might choose the position of the brightest point along axis 2--2 as the target point of the beam as it crossed axis 2--2. Accordingly, one might simply scan the 480 data values to determine which is largest and assume that the point along axis 2--2 corresponding to the largest intensity data value is the point of maximum intensity along the axis. In the example of FIG. 2, the data value corresponding to the vertical axis marked "peak" is near the actual peak of the waveform distribution. However, when noise causes a spike in the intensity distribution of the beam, the spike may strongly influence the apparent intensity maximum. Also the value of the peak determined by this method depends on the relative positions of the capacitors in the CCD with respect to the waveform display such that peaks of repetitive cycles of a periodic waveform may not appear to have consistent values. Finally, the assumption that the intended target point of the beam occurs at the point of maximum intensity along each vertical axis on the screen is usually inaccurate. In an analog oscilloscope, calibration of vertical and horizontal position of a trace is often performed manually and an operator performing the adjustment may not set the brightest portion of the beam at the target point on the screen. In fact, in a poorly calibrated oscilloscope, no part of the beam may actually strike the intended target point.
Other methods of digitizing the trajectory of the target point of the beam across the screen assume that the beam is aimed at points corresponding to the median or centroid of the light intensity distribution along each vertical axis on the screen. The "median" of the light intensity distribution of FIG. 2 is the point at which the areas under the curve on either side of the point are equal. The "centroid" of the distribution is its "center of mass". The centroid and median of the distribution of FIG. 2 are as shown and algorithms for finding the median or centroid of a curve are well known. When an intensity distribution along a single vertical axis is Gaussian, i.e. symmetrically bell-shaped, the centroid and the median both appear at its peak. However when the distribution is asymmetrical, as shown in FIG. 2, the centroid and median appear other than at the peak. In the example of FIG. 2, the maximum data value is closer to the actual peak than the median or centroid, but as the distribution becomes more "Gaussian" the centroid and median move toward to the distribution peak. Use of the centroid or median methods is usually advantageous over the peak intensity method because the target point of the waveform as determined by the centroid or the median method is not as strongly affected by noise as the position as determined by peak intensity and is not as strongly affected by the position of capacitors of the CCD with respect to the waveform display.
However, while the median and centroid methods provide fairly consistent results for a distribution along axis 2--2 of FIG. 1, these methods produce undesirable results for a distribution such as along axis 3--3 of FIG. 1, which distribution is shown graphically in FIG. 3. The distribution of FIG. 3 has left and right bell-shaped areas 18 and 20 corresponding to the top and bottom peaks of the waveform of FIG. 1 where intersected by axis 3--3, and a central area corresponding to the trailing edge of the waveform wherein the intensity of the trace is negligible. In this case the centroid is in the low intensity area between the peaks and the median is at a low intensity point near the rightmost side of area 18. It would be preferable to assign the target point of the waveform nearer the point corresponding to the center of larger area 18 so that a subsequently computer-generated waveform has more abrupt edges. Finally, the assumption that at any moment the beam was "aimed" at a target point on the screen corresponding to the median or centroid of the light intensity distributions along a vertical axis on the screen representing the current time is usually untrue. Thus the median and centroid methods, while more consistent than the maximum intensity method, are also inaccurate.